Quantifying the Ground-State Hydrogen-Bond Formation of a Super-Photoacid by Inspecting Its Excited-State Dynamics

Abstract

The identification and quantification of hydrogen (H)-bonded complexes form the cornerstone of reaction-mechanism analysis in ultrafast proton transfers. Traditionally, the Benesi-Hildebrand method has been employed to obtain the formation constants of H-bonded complexes, given that H-bonding additives induce an alteration in spectral features exclusively through H-bond formation. However, if the additive introduction impacts the bulk polarity of the solution, inducing a spectral shift, the spectroscopic method's accuracy in analyzing the H-bond formation becomes compromised. In this study, we scrutinize H-bond formation under the influence of an H-bond accepting solute in an aprotic solvent. This is achieved by quantifying the fractions of two concurrent pathways involved in the excited-state proton transfer (ESPT) of a super-photoacid: the ultrafast ESPT of an H-bonded complex vs. the diffusion-controlled ESPT of the free acid. Our method offers improved accuracy compared to conventional steady-state spectroscopic techniques, by directly quantifying the H-bonded complexes using the time-resolved spectroscopic method, thereby circumventing the aforementioned limitation.